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Durability of pva fibres in fibre-cement products
Citation for published version (APA):Lhoneux, de, B., Akers, S., Alderweireldt, L., Amiya, S., Carmeliet, J., Hikasa, J., Saenen, W., Studinka, J.,Tomka, I., & Vanden Bosch, M. (2002). Durability of pva fibres in fibre-cement products. In IVth InternationalSymposium on Concrete for a Sustanable Agriculture (pp. 275-284)
Document status and date:Published: 01/01/2002
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DURABILIТY STUDY OF PVA FIBRES IN FIBRE-CEMENT PRODUCTS
В. de Lhoneux1, 5. Akers2, L. Alderweireldt1, 5. Amiya3
, J. Carmeliet4, J. Hikasa3,
W. 5аепеп1, J. 5tudinka5, 1. Tomka6
, М. Vanden Bosch7
1 Redco NV, Kuiennansstraat, 1, В-1880 Kapelle-op-den-Bos, Belgium; emails: [email protected]; [email protected]; [email protected];
2 Etemit AG, СН-8867 Niederumen, Switzerland; e-mail: [email protected] 3 Kuraray Со, Ltd, 12-39, 1-Chome, Umeda, Кita-kч, Osaka 530-8611, Jарап;
emails: [email protected] & [email protected] 4 Katholieke Universiteit Leuven, Faculteit Toegepaste Wetenschappen, Dept.
Burgerlijke Bouwkunde, LаЬо Bouwfysica, Kasteelpark van Arenberg, 51, В-3001 Heverlee, Belgium; email: [email protected].Ьe
5 Polyfibre SA, с/о EASA SA rue de 1' Avenir, 42, L-1147 Luxemburg 6 Eidgenбssische Technische Hochschule, Dpt Werkstoffe Institut fiir Polymers, ЕТН zentrum,
Universitatstrasse, 41, СН-8092 Ziirich, Switzerland 7 Etemit NV, В-1880 Kapelle-op-den-Bos, Belgi um; email: marc. [email protected]
Abstract
In this study, results are presented conceming the long tenn behavior of PV А fibres iп fibre-cement products used for roofing and cladding applications. In а first part, we analyse the temperature and environment conditions under which PVA fibres сап degrade. lt is shown that а decrease of the fibre tensile strength does occur when fibres are immersed for more than 6 months in warm (>40°С) cement saturated solutions, while по sigпificant changes occur at lower temperatures iп the same period of time. Further, in order to assess the Jikelihood of such а degradation iп the fibre-cement matrix under outdoors weathering coпditioпs, we iпvestigate Ьу modelling and experimentation the hygrothermal behaviour of fibre-cemeпt slates exposed to mid European climate conditioпs. It is showп that high moisture coпtents are accompanied Ьу temperatures lower than 20°С. Microscopy analysis further reveals that fibres are well bound to the cement matrix allowiпg little micron size porosity around them. Fiпally, we investigate the physico-chemical properties of aged fibre-cement products as well as those of the fibres which were extracted from them. From these data, it is shown that fibres do not substaпtially degrade upon exposition of the investigated fibre-cement products to natural weathering.
1. lntroduction
New Techпology ("NТ') fibre-cement products have been produced for about 20 years Ьу differeпt European manufacturers for roofing апd cladding of private, agricultural as well iпdustrial buildings. The production technology is mainly the Hatscheck process, which is named after its Austrian iпveпtor. As far as roofing materials are concemed, the reinforcing fibres have Ьееп mainly polyvinyl alcohol (PV А) fibres. While the ageiпg behavior of these products under natural weathering conditioпs is fouпd to Ье satisfactory, по complete and systematic study of the ageing mechanisms related to а possiЫe degradation of these PV А fibres has been performed to date. Results from а preliminary duraЬility study were puЫished Ьу S. Akers et al. (1989). The preseпt research program was set-up Ьу а
276
workiпg group iпvolviпg fibre-cemeпt maпufacturers, uпiversities as we\l as а PV А fibre maпufa~turer iп order ~~ further investigate the loпg teпn behaviour of these building products, with emphas1s on the durab1\1ty of the fibre itse\f.
ln the first part of this research, we апа\уsе the temperature and enviroпment coпditioпs under which PV А fibres degrade. Further, in order to assess the likelihood of such а degradatioп iп the fibre-cemeпt matrix under outd~rs weatheriпg coпditioпs , we iпvestigate Ьу model\ing and experimentation the hygrotheпnal behavюur of slates exposed to mid European climare conditions, as well as the microstructure of the fibre-cemeпt interface. Finally: fibres were extracted from aged fibre-cement products and their physico-chemical propert1es were evaluated. Using the experimental and modelliпg results, we give evideпce that fibres iп slates exposed to weathering do поt substantially degrade.
2. Degradation of fibres in alkaline solutions
The degradatioп of ~VA fibres iп а hot cemeпt saturated solutioп was evaluated at Kuraray, Ltd (Japan~ and at the E1dgeпбssische Techпische Hochschule (ЕТН) (Switzerland) at 20, 40 and 60°С,
accordшg to experimeпtal coпditions w.hich will Ье puЫished elsewhere.
Although results do differ to some exteпt due to the somewhat differeпt test coпditions both studies iпdicate а fibre degradatioп after 6 moпths exposure, wheп the temperature exceeds 60~С (Kuraray) or 40°С (ЕТН) (Fig. 1 & ТаЫе I ).
1700 .--------------~
700 -r-------- --- --- --1
200 -г---т----г----.----.-~-~
2 6 12 18
months
~2О0
С
~4О0
С
-lr-60°C
;igure 1. Evolution of the teпsile strength of PV А fibres immersed in cemeпt saturated water in uпction of the time (experiment Ьу Kuraray, Ltd)
277
ТаЬ\е 1. Teпsile streпgth of PV А fibres immersed iп cemeпt saturated water for 6 months
(experimeпt Ьу ЕТН).
Reference 20°с
40°С
6О0С
Teпsile Streпgth (N/mm2)
1660 1694 1498 1397
Teпsile streпgth (%)
100 102 90 84
3. Simulation of the hygrothermal behaviour of the fibre-cement slates
The aim of the preseпt sectioп is to пumerically simulate the moisture апd temperature coпditions prevailiпg iп the fibre-cemeпt slates wheп exposed to middle European climatic conditions.
This requires first а determination of the basic physical properties of the slates, iпcluding open porosity, vacuum water saturatioп content, bulk density, moisture retention curve, pore size distributioп as well as liquid and vapour water penneaЬilities (Carmeliet апd Roels, 2001; CarmeJ
апd Roels, 2002).
Three maiп pore sizes were identified Ьу the mercury iпtrusion technique: а very fiпe size (10·9
to 1 о·8 m) associated with the cemeпt gel phase, an intennediate pore size of 10·
8 to 10·
7 m and а
coarse size 00·1 to 10·5 m) linked to various product inhomogeneities. The simulation itself was based оп the "Delphin4" calculatioп model of the Techпical University с Dresden (J. Gruпewald, 1997) al\owing to predict heat and mass traпsfers iп function of natural c\imatic conditions. Climate conditions choseп here were those ofNorth апd South Germaпy.
From this modelliпg work, it appears that the slate experieпces high mеап moisture сопtепt during the wiпter апd тапу dryiпg/wettiпg cycles duriпg the summer. Wettiпg mechaпisms are raiп, hygroscopic adsorption апd uпdercooliпg condensatioп. Dryiпg is опlу possiЫe Ьу water vapour diffusioп апd is therefore slower thaп wetting. Very fiпe (hygroscopic) pores are filled Ьу water about 80% of the time оп а yearly basis, while Iarger (capillary) pores are either never filled (сепtе of the slate) or опlу filled 20% of the time iп the top апd bottom layers ( 1 mm) of the slates at
temperatures < 20°С.
High moisture coпtents iп the slates are accompa11ied with temperatures Jower than 20°С (Fig. 2). For higher temperatures (>40°С), опlу hygroscopic pores with а radius < 10"
9m will Ье filled with
water. The cumulated time during which the slates are exposed to temperatures duriпg опе year is limited, i.e" 15 days at >40°С and 3 days at >6О0
С. In other words, wheп the temperature of the slate rises, it dries out, at Jeast at the top and bottom layers, and еvеп within the center of the slate, it remaiпs we\l be\ow the saturatioп point which is i1 this case 269 kg/mЗ. The comЬination of high moisture and high temperature which has been observed to Ье daпgerous for the staЬility of the fibre has thus а low level of probaЬility of occurreпce along the year. Additioпal\y, water is most of the time confined to the finest pores wher moisture traпsport is rather low (water vapour diffusion and adsorptioп iпstead of capillary transfc1 Under such coпditions. water hased chemical degradation kiпetics аге assнmed to he Yerv slr)\v.
L/~
reference В, topside
·20 -10 10 20 30 40 50 60 70 temperature (0С)
reference В, middle
·20 ·10 о 10 20 30 40 50 60 70 temperature (0С)
Figure 2. Simulated moisture content distribution over one year at the top side ("reference В, topside") and in the middle ("reference В, middle") of а slate as а function of the temperature at the respective positions, for weather conditions coттesponding to middle Germany.
4. Microscopical investigation of the fibre-cement interface
From the preceding section, it appears that the роге size distribution shows three main subsystems. The question which is the object of the present section is to know whether significant amounts of large pores (micron size), where water and associated ioлs сап move most easily, сап Ье found in the vicinity of the fibres.
This was done applying an image analysis technique to SEM pictures of slate cross sections recorded in the backscattered mode. This allows to quantify Lhe relative amounts of different pha~ pores (> 1 µm diameter), fibres, hydrated cement gel, unhydrated clinker particles (W. Saenen et <
1995).
From the results shown in Fig. 3, it appears that the amount of micron size pores in unaged slates rather low (4 to 7 volume %) and further reduces with ageing (2 to 3 vol. %) due to carbonatation and hydration reactions. Further, such Jarge pores are either evenly distributed or even less preseг in the vicinity of the PVA fibres than at other Jocations. This сап presumaЫy Ье explained Ьу the high affinity of the fibre surface for the hydrated cement gel. The fibres are thus well encapsulate Ьу the fine porous mineral phase and are therefore not exposed to high degrees of moisture saturation.
"8- КОВ REF
2 Б 10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 74 78
Distance to fibre surface [µm]
Figure 3. Percentage porosity in the vicinity of PV А fibres. КОВ REF = unaged slate from Belgi1 КОВ 19У = slate from Belgium after 19 years outdoors weathering, NU REF = unaged slate fron Switzerland.
5. Natural and accelerated weathering: fibre and fibre-cement properti
In this Iast part, we analyse the properties of differeлt fibre-cement products as well as of fibres extracted from them which have been exposed to natural as well as to accelerated ageing conditir Aim is to analyse whether fibre degradation has occurred.
2~U
5.1. Natural weathering
Corrugatcd shecLS, slatcs and far;ade sheeLs were sampled from ageing sites in Switzerland and in Belgium after up 10 .18 years exposure and Lheir bending s1reлg1h was compared 10 1he original values. Corrugated shee1s are produced \vi1h or \ViLhout post compression алd their density ranges betwccn 1.60 10 1.75 g/cmЗ in the .first case and 1.4 and 1.55 g/cm3 in the second one. They are referred herea.fter as l1igh density and medium density sheets. As appears from fig. 4, the strength of the slates алd of the fac;ade sheeLS was found 10 Ье higher than before ageing, while for COJтugated sheets, the strength \Vas either unchanged (medium density sheets) or somewhat (- 5%) reduced (h igh deлsity sheets). From physico-chemical analyses of the products, it appeared that they all had increased iл density, \Vhile 1I1eir degree of hydration as well as of carbonatation had similarly incrcased.
Product embriltlement (toughness reduction) with age was evident and сап Ье related to the matrix changes алd corresponding increase of the fibre-cement interfacial bond.
As appears from ТаЬ!е 2, the properties of the fibres which were extracted from the sheets did лоt indicate any significant change as compared to the original values.
ТаЬ!е 2. Tensile strength, crystallinity and molecular weight of PV А fibres extracted from naturally aged and from unaged products
FiвRE PROPERTIES
14 yrs aged medium density corrugated sheet Belgium 15 yrs aged high density corrugated sheets Switzerland 18 yrs aged roofing slates Belgium 16 yrs aged roofing slates Switzerland 12 yrs aged fш;ade panels Switzerland Unaged roofing slate Switzerland
5.2. Accelerated ageing
Tensile Strength N/1111112
1180 1240 1100 1140 1270 1200
Crystal- Mol. weight linity %
56 141.000 54 133.000 58 153.000 58 141.000 58 143.000 55 146.000
The first accelerated ageing test consisted in а wet/dry cycl ing in а СО2 rich environment, ("];!_eschleu11igt Дlteruлg ~02 ', or "ВАС test) with 1he follo\ving sequcnce: 1) samples placed under \Vater at room 1empcra1ure (6 hr), 2) drying а1 60°С (0.5 hr), 3) cooling \Vith amЬient air (0.5 hr), 4) СО2 enriched a.ir (25%) at room tempcrature (Зhг), 5) drying at 60°С (3 hr), 6) cooling with ventilated amЬient air (1 hr).
N
Е Е -z
А)
~ ~
"С nl о
...J
..: а:а
В)
281
30
25
~ 20 -~дд
д -6 :t!: 15 ... ~ 10 -1 1
о 5 10 15
Years of exposure
120
100 ~" v
80 . QEj _ в
60
40
20 - - ---о
о 10 20
Уеа rs of exposition
oSL NU
дSL КОВ
<> FS NU
1
20
<> СН pressed
о NL non pr.
Figure 4. Fibre-cement strength evolution during natural weathering in Belgium and Switze1 А) Roofing slates алd. fa1Jade shcets Пexural strength; SL NU = slates/Switzerland; SL КОВ slates/Вelgium; FS NU = fa!Jade sheets/Switzerland . В) Corrugated sheets breaking load; "СН pressed" = high density sl1eets/ \vitzerland; "NL n = medium density sheets produced in the Netherlands and exposed i-n Belgium. The breakin of the high density sheet before weathering is taken as the 100% reference.
А series of medium and high density sheets were made using а pilot machine which simulates the Hatscheck process for fibre-cement production. They were submitted to 1 ООО cycles of this test and the evolution of Lheir properties as well as of those of the fibres extracted from them in function of the number of cycles is summarized in ТаЫе 3.
ТаЫе 3. Fibre-cement and PV А fibre properties during accelerated ageing test (ВАС). FI. Str.= Flexural streпgth, Poros.= total porosity; Hydr.= degree of hydration; Carb.= degree of carbonation, Т. Str. = Tensile strength, Mol. Wt = Molecular weight.
# cycles PRODUCT FI. Str. Poros. Hydr. Carb.
N/mm2 (%) (%) (%)
High density sheets о
50 200 500
1000
Medium density sheets
22.0 27.4 33.2 29.0 30.4
о 16.3 50 19.3
200 20.7 500 20.4
1000 20.3
12.0 10.0 10.1 9.&
18.0 17.8 12.2 10.1
47 58 66 74
48 45 66 64
8 53 66 73
6 51 91 81
FIВRE
Т. Str. Mol. Wt N/mm2
1130 143.000
1290 138.000 1060 148.000
1180 157.000
1270 142.000 1130 166.000
As сап Ье seen from these data, the strenglh ofthe products first increases with the number of cycles and then staЬilizes. Physico-chemical analysis of Lhe aged products did reveal the same Lrends as observed in natural ageiпg, i.e., an increase iп deпsity, degree of hydration and carbonation. No sigпificant changes were observed iп the fibres properties, at least up to 500 cycles. The physicochemical tests (degree of hydratioп, and carboпatatioп, porosity) оп the samples submitted to the 1000 cycles and оп the fibres extracted from them have поt yet Ьееп carried out. However, we found already that Lhe strength of the sheets for these 1 ООО cycles did not change in comparison with 500 cycles.
In а second test series, samples were submitted to the "warm water test" of the CEN 494 standard but prolonged up to 200 days instead of the prescribed 56 days. In this test, the products are left under water at 60°С until the end of the test. Here again, the fibre properties after the test were not different from the properties of fibres extracted from the unexposed products (ТаЫе 4).
During this test, the degree of hydration and of carbonatation did not increase as much as in the ВАС test and the porosity tended to increase rather than to decrease as in the ВАС test or in the natural ageing. This results most likely from some fonn of leaching during the test.
ТаЫе 4. Fibre-cement апd PV А fibre properties during exposure to the extended CEN warm V.'a test. Fl. Str.= Flexural streпgth, Poros. = total porosity, Hydr. = degree of hydration, Carb. = deg of carbonatation, Т. Str. = Tensile Strength; Mol. Wt = molecular weight.
Time Product Fibre
(days) FI. Str. Poros. Hydr. Carb. Т. Str. Mol.Wt
N/mm2 о/о о/о о/о N/mm2 о/о
Нigh density sheets о 22.0 12.0 47 8 1130 143.000
56 23.6 12.6 51 25 1110 146.000
112 23.3 12.8 54 21 1230 175.000
200 23.1 12.4 53 23 1160 158.000
Medium deпsity sheets о 16.3 18.0 48 6 1180 157.000
56 16.4 18.1 45 32 1120 150.000
112 16.5 18.5 47 36 1240 158.000
200 15.4 18.7 46 38 1160 157.000
6. Conclusions
Although it appears that long term exposition of PV А fibres to hot alkaline solu~ions decreases their tensile strength, по signs of degradation could Ье observed on the fibres wh1ch were extract from roofing and claddiпg fibre-cement products exposed to outdoors weathering for up to 18 yt
in Belgium and Switzerland.
It is hypothesized that the following factors do explain this:
high temperatures in the exposed sheets are only accompanied with low moisture content~. The fibres are well embedded in the fine porous cement matrix with а small amount of m1cr(
size porosity. The cumulated yearly time during which the product temperature exceeds 40°С does not ехс 15 days under the mid European conditions.
Such PV А fibre-cement products сап thus Ье considered as duraЫe.
References
1. Akers, S., Studinka, J., Meier, Р. , Dobb, М., Johnson, D., Hikasa, J. (1989). Long term duraЬility of PVA reinforcing fibres in а cement matrix. Intemational Joumal of Cement Composites and Lightweight Concrete, 11, 2, 79-91 .
2. Carmeliet J ., Roels, S. (2001 ). Determination of the isothermal moisture transport properties of porous building materials, Journal of Thermal Envelope and Building Science, 24, 183-21 О.
3. Carmclict J ., Roels, S. (2002) Dctcrmination of the moisture capacity of porous building materials, Joumal ofThermal Envelope and Building Science, iп press.
4. Grunewald, J. (1997) Diffusiver und konvektiver Stoff- und Energietransport iп kapillarporбse Baustoffe, PhD thesis ТU Dresden, Germany
5. Saenen, W., Van Oyen Р., Van der Heyden, L. (1995). In: J. Elsen (Ed.): Quantitative approach to hydration in fibre-cement products using automated SEM. Proc. Fifth Euroseminar оп Microscopy Applied to Building Materials, Wetenschappelijk ел Technisch Centrum voor het Bouwbedrijf, Brussel, рр. 89-95
Concrete and environment
